1. Field of Invention
The present invention is directed to a digital driver circuit suitably used to drive an electro-optical device, such as a liquid-crystal device, using a TFT active matrix drive scheme, an electro-optical device having the digital driver circuit, and an electronic apparatus having the electro-optical device. More particularly, the invention is directed to a digital driver circuit or the like which receives a digital image signal as an input and uses multi-ramp waves to generate an analog drive signal.
2. Description of Related Art
Digital driver circuits which receive a digital image signal as an input and drive a display panel such as a liquid-crystal panel, to create a gradation display are known. For example, a digital driver circuit including an SC-DAC (Switched Capacitorxe2x80x94Digital to Analog Converter) circuit for selectively performing charge sharing or charge pumping of charges that accumulate in a plurality of capacitors having different capacities by a switching element depending on a digital image signal to generate plural voltage levels have been used. The SC-DAC circuit outputs plural voltage levels to a signal line of the display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be realized. Typically, the digital driver circuit of the form including the SC-DAC circuit is externally connected to a display panel.
As another example of a digital driver circuit for driving a display panel such that a gradation display can be generated, a digital driver circuit of a form including a serial divided-voltage resistor circuit is disclosed in Japanese Unexamined Patent Publication No. 9-54309. In this form, a serial divided-voltage resistor circuit divides a plurality of reference voltages depending on a digital image signal to generate plural voltage levels, and the voltages are output to the signal line of a display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be created.
In addition, as another example of a digital driver circuit for driving a display panel such that a gradation display can be generated, a digital driver circuit having a form including a PWM (pulse width modulation) circuit and using a ramp wave (saw-tooth wave) voltage is disclosed in Japanese Unexamined Patent Publication No. 9-244588. In this formation, the digital image signal is subjected to pulse width modulation by the PWM circuit to generate pulse signals having pulse widths corresponding to the digital image signals. A ramp wave is selected on a time axis depending on the pulse width to generate plural voltage levels, and the voltages are output to the signal line of the display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be created.
In a digital driver circuit of this type, a general demand for a simplified circuit arrangement or for low power consumption is strong, and at the same time, a demand for drive performance to be high even with an increase of the size of a display panel. In particular, xcex3-correction required depending on a non-linear gradation characteristic for drive signal voltages in a display panel, such as a liquid-crystal panel, must be accurately performed by a circuit arrangement that is as simple as possible.
However, according to a digital driver circuit of a form including the conventional SC-DAC circuit, in order to make drive performance high, a large-capacity capacitor is required. For this reason, for example, the digital driver circuit is actually limited to driving a liquid-crystal panel having a size of about 5xe2x80x3 in diagonal length. More specifically, it is difficult for the digital driver circuit to drive a display panel, such as a liquid-crystal panel, having a size larger than the above size. In particular, in a display panel having a digital driver circuit built therein, this formation in which a large capacitor must be formed on a substrate is not appropriate from the viewpoint of total circuit area or pixel pitch.
According to the conventional digital driver circuit including a serial divided-voltage resistor circuit, in order to improve drive performance, power consumption in each resistor inevitably increases with an increase in current. For this reason, the digital driver circuit cannot have a low power consumption. At the same time, a switching element, such as a thin film transistor, for performing switching control of the resistors must be increased in size to improve drive performance, and the area of the entire circuit increases. In particular, in a display panel having a digital driver circuit being built therein, a large number of resistors and large-size thin film transistors or the like must be formed on a substrate, which is not appropriate from the viewpoint of circuit area or pixel pitch.
In addition, according to the conventional digital driver circuit of a form including a PWM circuit, control of the voltage of a ramp wave with respect to time must be extremely accurately performed to correctly generate a gradation display. Therefore, an amplifier for supplying a ramp wave requires high performance, must be able to rapidly saturate a voltage for a signal line at a correct timing depending on a pulse signal, and also must create a highly accurate waveform for the ramp wave. Practically, it is very difficult to realize this type of circuit. Since a large-power ramp wave must be input at a low output impedance to improve drive performance, a problem that power consumption in the digital driver circuit is considerably high occurs. In particular, when y-correction for a digital image signal is required, problems can occur. More specifically, when as a scheme of xcex3-correction, any one of (i) a scheme in which the duty of a PWM base clock is changed for a gradation level depending on the characteristics of a display panel, (ii) a scheme in which a ramp wave for a time axis is changed into an S shape depending on the characteristics of a display panel, and (iii) a scheme in which a pseudo-S-shaped ramp waveform depending on the characteristics of a display panel is formed by a voltage which exactly gradually changes is selected, a voltage must be controlled at an accuracy that is higher than that used when xcex3-correction is not performed. Therefore, it is practically almost impossible that voltages for driving a plurality of signal lines are. assured by the digital driver circuit of this formation. For this reason, the digital driver circuit of this formation is not typically used.
The present invention has been made in consideration of the problems described above, and has as its problem to provide a digital driver circuit having relatively low power consumption and relatively high performance, an electro-optical device having the digital driver circuit, and an electronic apparatus having the electro-optical device.
The invention provides a digital driver circuit that receives a digital image signal of n (n is a natural number not less than 2) bits as an input and generates an analog drive signal corresponding to the digital image signal to output to a signal line of an electro-optical device. The digital driver circuit includes a series selection circuit that selects one series from plural series of standard multi-ramp waves having voltages which change in steps with the passage of time depending on the value of y (y is a natural number) bits of the n bits. A time selection circuit selects, on a time axis, a voltage which changes in steps in at least the selected series of standard multi-ramp waves depending on the value of x (x is a natural number) bits whose bit position is higher than that of the y bits of the n bits. The drive signal is output based on the selected voltage in the selected series.
One series is selected from the plural series of standard multi-ramp waves depending on the value of the y bits (e.g., medium or least significant three bits, four bits, or the like) of the n bits (e.g., six bits, 8 bits, 16 bits, or the like). On the other hand, the voltage which changes in steps in at least the selected series of standard multi-ramp waves is selected on the time axis depending on the value of the x bits (e.g., most significant three bits, four bits, or the like) whose bit position is higher than the y bits of the n bits. The series selection and the voltage selection may be simultaneously performed, or one of them may be performed first. When the series selection and the voltage selection are combined with each other as described above, voltages (i.e., drive signals) corresponding to the values of digital image signals are generated. For this reason, the step-form change in voltage in each of the series of standard multi-ramp waves is a relatively large change at every step, and the change in voltage at each step is maintained over a relatively long time period. Therefore, the accuracies of timings required for the series of standard multi-ramp waves become considerably low. In addition, even if the performance of the amplifier for supplying the standard multi-ramp waves is low, a time margin which is sufficient to saturate a signal line with the voltages of drive signals can be assured.
More specifically, when a drive signal is generated by using a constant voltage (saturation voltage) which is achieved after each ramp wave rises without using a voltage at the leading edge of the ramp wave, a sharp rising characteristic with respect to the corresponding ramp wave is not necessary. As a result, drive performance of the digital driver circuit can be improved by using a circuit having a relatively low through rate while the power consumption is made low, and compensation for temperature or the like can also be easily performed. Furthermore, such a circuit can be formed having a relatively small circuit area and a relatively simple structure. Therefore, the present invention is applied as a digital driver circuit, having high drive performance, for driving an electro-optical device such as a large-size display panel, or a digital driver circuit which can be built in an electro-optical device and has a small size and low power consumption.
In one aspect of the invention, a time selection circuit includes a PWM circuit for generating pulse signals having different pulse widths depending on the value of the x bits, and a first switching circuit for selecting the voltage on a time axis depending on the pulse widths. The series selection circuit includes a decoder for decoding the value of the y bits and a second switching circuit for selecting the series depending on the decoded value.
Pulse signals having different pulse widths are generated by the PWM circuit depending on the value of the x bits first, and, depending on the pulse widths, a voltage which changes in steps in the standard multi-ramp waves is selected on the time axis by the first switching circuit, e.g., a thin film transistor. On the other hand, in the series selection circuit, the value of the y bits is decoded by the decoder, and, depending on the decoded value, a series of standard multi-ramp waves is selected by the second switching circuit, e.g., a thin film transistor. Therefore, selection of standard multi-ramp waves and selection of a voltage can be performed with high reliability by using a combination of the PWM circuit, the decoder, and the switching circuit. When this arrangement is employed, high drive performance can also be realized while suppressing power consumption to a low level.
In one aspect of the invention, the selected voltage in the selected series is output as the drive signal. Thus, a selected voltage in the selected series of standard multi-ramp waves can be directly output as a drive signal. Therefore, when the number (n) of bits of a digital image signal is small, i.e., about six, a voltage is selected on the time axis depending on, e.g., three high-order bits, and a series of standard multi-ramp waves is selected depending on three low-order bits. In this manner, the digital driver circuit is especially effective from the viewpoint that a simple circuit arrangement and a simple selection scheme can be used.
In one aspect of the invention, the digital driver circuit includes a voltage change circuit that changes the selected voltage in the selected series depending on the value of z (z is a natural number) bits whose bit position is lower than that of the y bits of the n bits, and the changed voltage is output as the drive signal.
The selected voltage in the selected series of standard multi-ramp waves is changed by the voltage change circuit depending on the value of the z bits (e.g., least significant three bits, four bits, or the like) whose bit position is lower than that of the y bits. The changed voltage is output as a drive signal. Therefore, when the number (n) of bits of a digital image signal is large, i.e., about eight, a voltage is selected on the time axis depending on three high-order bits, a series of standard multi-ramp waves is selected depending on two medium bits, and the selected voltage is slightly changed depending on three low-order bits. In this manner, the digital driver circuit is effective from the viewpoint that multi-gradation can be realized with low power consumption and high drive performance.
In one aspect of the invention, the voltage change circuit includes an SC-DAC circuit for increasing and decreasing the selected voltage in the selected series depending on the value of the z bits, the series selection circuit for further selecting one series of plural series of reference multi-ramp waves for causing the SC-DAC circuit to increase and decrease the selected voltage, and the time selection circuit for further selecting, on a time axis, a voltage which changes in steps in at least the selected series of reference multi-ramp waves depending on the value of the x bits.
One series of the plural series of reference multi-ramp waves for causing the SC-DAC circuit to increase and decrease the selected voltage is further selected depending on the value of the y bits. On the other hand, in the time selection circuit, the voltage which changes in steps in at least the selected series of reference multi-ramp waves is further selected on the time axis depending on the value of the x bits. The series selection and the voltage selection may be simultaneously performed, or one of them may be performed first. In the voltage change circuit, the selected voltage in the selected series of standard multi-ramp waves is increased and decreased by the SC-DAC circuit depending on the value of the z bits. Therefore, when the number (n) of bits of a digital image signal is large, i.e., about eight, a voltage selected depending on three least significant bits is slightly changed by using the SC-DAC circuit. In this manner, the digital driver circuit is effective from the viewpoint that multi-gradation can be realized with low power consumption and high drive performance. In particular, the present invention, which performs only fine adjustment of the voltage of a drive signal by using the SC-DAC circuit, can considerably increase the limit of drive performance in comparison with the prior art in which all the gradation levels are realized by using an SC-DAC circuit. Therefore, the present invention is suitable as a digital driver circuit to be built in a display panel which generally has a limited size and has limited space for forming an excessively large capacitor therein.
In one aspect of the invention, the SC-DAC circuit performs charge sharing using a plurality of capacitors depending on the value of the z bits on the basis of the selected voltage in the selected series of standard multi-ramp waves and the selected voltage in the selected series of reference multi-ramp waves.
Charge sharing using a plurality of capacitors is performed by the SC-DAC circuit depending on the value of the z bits on the basis of the selected voltage in the selected series of standard multi-ramp waves and the selected voltage in the selected series of reference multi-ramp waves. Therefore, a voltage between the voltage of the standard multi-ramp waves and the voltage of the reference multi-ramp waves corresponding to the standard multi-ramp waves can be output by charge sharing.
In one aspect of the invention, the voltage change circuit further comprises an inversion circuit that inverts the value of the z bits to input the value to the SC-DAC circuit, and the SC-DAC circuit performs voltage subtraction by charge sharing depending on the inverted value of the z bit.
The value of the z bits is inverted by the inversion circuit first, and the inverted value of the z bits is input to the SC-DAC circuit. At this time, in the SC-DAC circuit, voltage subtraction is performed by charge sharing depending on the inverted value of the z bit. Therefore, the voltage between the voltage of standard multi-ramp waves and the voltage of reference multi-ramp waves which correspond to the standard multi-ramp waves and is lower than the voltage of the standard multi-ramp waves at the same time can be output by voltage subtraction. In this manner, when the voltage of the reference multi-ramp waves is set to be lower than that of the standard multi-ramp wave, the reference multi-ramp waves in the digital driver circuit can be easily handled, and an amplifier having low performance can be advantageously used to generate the reference multi-ramp wave.
In one aspect of the invention, the SC-DAC performs charge pumping using a plurality of capacitors depending on the value of the z bits on the basis of the selected voltage in the selected series of standard multi-ramp waves and the selected voltage in the selected series of reference multi-ramp waves.
Charge pumping using a plurality of capacitors is performed by the SC-DAC circuit depending on the value of the z bits on the basis of the selected voltage in the selected series of standard multi-ramp waves and the selected voltage in the selected series of reference multi-ramp waves. More specifically, for example, the difference between the potential of the selected series of reference multi-ramp waves is added to the potential of the selected series of standard multi-ramp waves by a selected capacitor. Therefore, a large voltage can be applied with a small capacitance by charge pumping. For this reason, the capacitors can be reduced in size, so that an area occupied by the circuit can be reduced.
In one aspect of the invention, voltages of the plural series of standard multi-ramp waves increase or decrease every predetermined time unit in a period in which the voltages steadily increase or decrease in steps. The magnitudes of the voltages of the plural series of standard multi-ramp waves in the same time unit are constant in all time units in the period, and the maximum value of the voltages of the plural series of standard multi-ramp waves in one time unit is set to be smaller than the minimum value of the voltages of the standard multi-ramp waves in another time unit following the corresponding one time unit.
In the plural series of standard multi-ramp waves, a voltage having discrete values at predetermined intervals properly appears in one of the time units of one of the plural series of standard multi-ramp waves. For this reason, when a series of standard multi-ramp waves is selected, and the voltage of the selected series is selected on a time axis, a voltage which efficiently has discrete values can be obtained. The voltage is directly output as a drive signal, or a multi-gradation-level drive signal can be output on the basis of the voltage.
In one aspect of the invention, a multi-ramp wave generation circuit generates the plural series of standard multi-ramp waves.
The plural series of standard multi-ramp waves are generated by the multi-ramp wave generation circuit arranged in the digital driver circuit. Therefore, in particular, standard multi-ramp waves are not required to be supplied from an external circuit. The digital driver circuit including an SC-DAC circuit may further include a reference multi-ramp wave generation circuit that generates plural series of reference multi-ramp waves. The digital driver circuit may also be formed such that one or both of the standard multi-ramp waves and the reference multi-ramp waves are supplied from outside of the digital driver circuit.
In one aspect of the invention, the multi-ramp wave generation circuit adjusts the voltages of the plural series of standard multi-ramp waves to perform xcex3-correction of the digital image signal to the electro-optical device.
The voltage of the plural series of standard multi-ramp waves are adjusted by the multi-ramp wave generation means to perform xcex3-correction of the digital image signal to the electro-optical device, such as a display panel. In this case, the step-form changes in voltages of the series of standard multi-ramp waves are changes which increase in steps over a relatively long time period. For this reason, when the xcex3-correction is to be performed, the accuracies of timings required for the standard multi-ramp waves can be low. Therefore, by using a multi-ramp wave generation circuit having a relatively low through rate, xcex3-correction can be performed at a high accuracy while keeping power consumption low and improving drive performance.
In one aspect of the invention, the voltages of the plural series of standard multi-ramp waves are adjusted to perform xcex3-correction of the digital image signal to the electro-optical device, such as a display panel. In this case, the step-form changes in voltages of the series of standard multi-ramp waves ares changes which increase every step and the voltages at each step are maintained over a relatively long period of time. For this reason, when xcex3-correction is to be performed, the accuracies of timings required for the standard multi-ramp waves may be low. Therefore, by using a multi-ramp wave generation circuit having a relatively low through rate, xcex3-correction can be performed at a high accuracy while keeping power consumption low and improving drive performance.
In one aspect of the invention, an electro-optical device includes a digital driver circuit of the invention. Since the electro-optical device includes a digital driver circuit of the present invention, a large-size electro-optical device can be realized with low power consumption.
In one aspect of the invention, the electro-optical device is a liquid-crystal device having a thin film transistor as a switching element in each pixel and using a TFT active matrix drive scheme. The series selection circuit and the series time selection circuit are thin film transistors, respectively.
Since the series selection circuit and the series time selection circuit in the digital driver circuit for driving a liquid-crystal device using the TFT active matrix drive scheme are constituted by thin film transistors, respectively, various elements and circuits can be constituted by thin film transistors in the device. For this reason, the electro-optical device is advantageous in structure. In particular, such a digital driver circuit can be formed as a relatively simple circuit formed on a TFT matrix substrate, using thin film transistors, and having a relatively small circuit area. Thus, a liquid-crystal device which has a large screen, but low power consumption and uses the TFT active matrix drive scheme can be realized. In addition, when the digital drive circuit is formed such that the voltages of standard multi-ramp waves are adjusted to perform xcex3-correction, a multi-gradation-level, high-quality display operation can be performed while performing xcex3-correction at a high accuracy.
In one aspect of the invention, an electronic apparatus, such as a television set, a satellite navigation system, an electronic organizer, or a portable telephone set which has a large size and low power consumption and includes the electro-optical device according to the invention can perform a multi-gradation-level, high-quality display operation or the like.
The invention also provides a method for driving an electro-optical device. A series of standard multi-ramp waves, having voltages that change in steps with a passage of time, is selected based on y bits of an n bit digital image signal. A voltage in the selected series of standard multi-ramp waves is then selected based on x bits of the digital image signal. The x bits of the digital image signal have a higher bit position than the y bits. An analog drive signal is output to an electro-optical device based on the selected voltage.
The operations and other aspects of the present invention will be apparent from the embodiments to be described below.